zircons

“Instead, the chemical make up of the Jack Hills crystals suggests that they formed in the presence of liquid water, likely even an ocean. These crystals provide evidence that even the very early Earth was cooler and wetter than scientists used to think. A gentler Hadean could have permitted life to evolve far earlier in the planet’s history than scientists originally supposed.”
“Among the first important discoveries, says Watson, came out in 2001. Analysis of the relative amounts of different isotopes of oxygen indicated that the ratio was skewed toward “heavy” oxygen-18, as opposed to the more common “light” oxygen-16. “When a geologist sees a heavy oxygen signature in rocks,” said Watson, “it’s commonly understood to be a sign that the rocks formed in cool, wet, sedimentary processes at the Earth’s surface.” Thus, the magma that eventually gave rise to the zircons might have been formed from what had once been sediments deposited on the floor of an ancient ocean.”

“Scientists are beginning to imagine a Hadean world that experienced far more dramatic swings in surface conditions than anything revealed in subsequent geologic history. Most of the time there appears to have been a stable, liquid water ocean, perhaps ice covered.”